Semiconductor lasers may utilize any of a variety of different designs, the selected design typically driven by the requirements of the intended application. Unfortunately it is often difficult to find a device design that meets all of a specific application's requirements since many times one device characteristic, such as output power, may influence another device characteristic, such as beam quality. For example, ridge-waveguide (RW) lasers offer nearly diffraction limited beam quality but are only able to achieve output powers of approximately 1 watt. Conversely, broad-area lasers are able to reach high output powers, on the order of 20 watts, but suffer from poor beam quality.
One approach that has been investigated recently as a means of achieving both high output power and acceptable beam quality is the use of a master oscillator with a power amplifier (MOPA). In this type of system, the output from a single mode laser is injected into a power amplifier. These two components may be separate or combined on to a single device, the latter approach eliminating many of the alignment difficulties associated with the former approach.
FIGS. 1 and 2 illustrate a conventional flared semiconductor laser integrated on to a single substrate, this structure utilizing two separate electrical contacts. FIG. 1 provides a perspective view while FIG. 2 provides a top view of the same device 100. Semiconductor laser 100 is comprised of two components; a single mode ridge waveguide (RW) laser 101 that is coupled at one end to a high power flared section 103. RW 101 may be either a gain-guided structure or an index-guided structure. The non-coupled end surface 105 of RW 101 is typically coated with a high reflectivity (HR) coating. Alternately, RW 101 may utilize a distributed Bragg reflector (DBR) or a distributed feedback (DFB) section in which case surface 105 is coated with an anti-reflection (AR) coating. The outer, flared edge 107 of section 103 is typically coated with an AR coating.
To date, high power broad area lasers with a 100 μm aperture have been limited to around 10-15 watts of power with a beam quality factor, M2, of 1 in the transverse direction and an M2 of 15 (beam parameter product (BPP) of approximately 17 mm-mrad) in the lateral direction. In the recent past, tapered lasers utilizing the design shown in FIGS. 1 and 2 have shown higher beam quality than non-tapered broad area lasers, achieving a beam propagation ratio, M2, of 1.2 at output powers of up to about 5 watts in continuous (CW) operation. However, at higher output powers the beam quality in the lateral direction begins to experience degradation. This degradation is due to phase front distortion arising from optical feedback and subsequent gain-index coupling. Additionally, at high output power beam quality degradation in this type of laser can be caused by thermal gradient-induced index of refraction variations.
Accordingly, what is needed is a means for reducing beam quality degradation at high output powers in semiconductor lasers utilizing a tapered design. The device structure of the present invention achieves these goals.